Transmission component, method of manufacturing the same, and tapered roller bearing

ABSTRACT

A transmission component is incorporated into a transmission in which an input shaft, an output shaft, or a gear is rotatably supported by a rolling bearing. The component has a nitriding layer at a surface layer and an austenite grain with a grain size number falling within a range exceeding 10. This provides a transmission component having an increased anti-crack strength, enhanced dimensional stability, and a long fatigue life. A method of manufacturing such a transmission component and a tapered roller bearing are also provided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a transmission component havinga long rolling contact fatigue life, an increased anti-crack strength,or a reduced secular variation in dimension, and to a method ofmanufacturing the same and a tapered roller bearing.

[0003] 2. Description of the Background Art

[0004] To increase a bearing's components in life, a thermal treatmentis performed. Specifically, for example, in quenching the componentsthey are heated in an ambient RX gas with ammonium gas fartherintroduced therein to carbo-nitride their surface layer portion, forexample as disclosed in Japanese Patent Laying-Open Nos. 8-4774 and11-101247. This carbonitriding process can harden the surface layerportion and generate retained austenite in a microstructure to provideincreased rolling contact fatigue life.

[0005] The above-mentioned carbonitriding process is a process todiffuse carbon and nitrogen. This requires a high temperature maintainedfor a long period of time. As such, for example a coarsened structureresults and increased anti-crack strength is hardly obtained.Furthermore, as more austenite is retained, secular dimensionalvariation rate increases, which is also a problem in this carbonitridingprocess.

[0006] Against rolling fatigue, an increased life can be ensured, anenhanced anti-crack strength provided and an increased seculardimensional variation avoided by relying on designing a steel alloy toprovide an adjusted composition. Relying on designing the alloy,however, increases source material cost disadvantageously.

[0007] Future bearings in a transmission will be used in environmentsexerting large loads at high temperatures. In addition, they will berequired to be employed in a downsized transmission. Therefore, thebearings will be required to be operable under larger loads at highertemperatures than conventional. As such, there is a demand for a bearinghaving large strength, long life against rolling contact fatigue, andlarge anti-crack strength and dimensional stability.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide atransmission component having an increased anti-crack strength, anenhanced dimensional stability, and a long fatigue life (or a longrolling contact fatigue life in the case of a tapered roller bearing orwhen the component is a rolling bearing or a rolling bearing'scomponent), and a method of manufacturing the same and a tapered rollerbearing.

[0009] The present invention provides a transmission componentincorporated into a transmission capable of changing a rotational speedof an output shaft relative to a rotational speed of an input shaft bymeans of mesh of toothed wheels. The component has a nitriding layer ata surface layer and an austenite grain with a grain size number fallingwithin a range exceeding 10.

[0010] In the transmission component according to the present invention,a small austenite grain size allows significantly increased anti-crackstrength, dimensional stability and fatigue life (or rolling contactfatigue life when the component is a rolling bearing or a rollingbearing's component). With the austenite grain size number of l0 orless, any remarkable improvement of the fatigue life is impossible andthus the grain size number is greater than 10, and preferably 11 orgreater. Although further finer austenite grains are desirable, thegrain size number exceeding 13 is usually difficult to achieve. Notethat an austenite grain of the transmission component does not varywhether it may be in a surface layer portion significantly affected asit is carbonitrided or a portion inner than the surface layer portion.As such, the surface layer portion and the inner portion will be set aspositions serving as subjects of the aforementioned grain number range.

[0011] The present invention provides another transmission componentincorporated into a transmission capable of changing a rotational speedof an output shaft relative to a rotational speed of an input shaft bymeans of mesh of toothed wheels. The component has a nitriding layer ata surface layer and a fracture stress value of at least 2650 MPa.

[0012] The present inventors have found that steel that is carbonitridedat a temperature exceeding an A₁ transformation point and then cooled toa temperature of less than the A₁ transformation point, and subsequentlyreheated to a range of temperature higher than the Al transformationpoint and is quenched, can be provided with a nitriding layer allowingthe steel to provide a fracture stress value of no less than 2650 MPa,which has conventionally not been achieved. A transmission componentsuperior in fracture stress to conventional and thereby larger instrength can thus be obtained.

[0013] The present invention provides a further transmission componentincorporated into a transmission capable of changing a rotational speedof an output shaft relative to a rotational speed of an input shaft bymeans of mesh of toothed wheels. The component has a nitriding layer ata surface layer and a hydrogen content of at most 0.5 ppm.

[0014] In the still another transmission component according to thepresent invention, embrittlement of steel attributed to hydrogen can bealleviated. If steel has a hydrogen content exceeding 0.5 ppm the steelhas reduced anti-crack strength. Such a steel is insufficiently suitablefor a support structure for a hub experiencing heavy loads. A lowerhydrogen content is desirable. However, reduction of the hydrogencontent to the one less than 0.3 ppm requires long-term heat treatment,resulting in increase in size of austenite grains and thus deteriorationin toughness. Then, a hydrogen content is desirably in a range from 0.3to 0.5 ppm and more desirably in a range from 0.35 to 0.45 ppm.

[0015] In measuring the above hydrogen content, diffusible hydrogen isnot measured and only the non-diffusible hydrogen released from thesteel at a predetermined temperature or higher is measured. Diffusiblehydrogen in a sample of small size is released from the sample to bescattered even at room temperature, and therefore the diffusiblehydrogen is not measured. Non-diffusible hydrogen is-trapped in anydefect in the steel and only released from the sample at a predeterminedheating temperature or higher. Even if only the non-diffusible hydrogenis measured, the hydrogen content considerably varies depending on themethod of measurement. The above mentioned range of hydrogen content isdetermined by thermal conductimetry. In addition, as detailed later, themeasurement may be taken by means of a LECO DH-103 hydrogen determinatoror like measuring device.

[0016] The above-described transmission component is preferably arolling bearing rotatably supporting the input shaft, the output shaft,or each of the toothed wheels, and the rolling bearing is a taperedroller bearing.

[0017] The above-described transmission component is preferably arolling bearing rotatably supporting the input shaft, the output shaft,or each of the toothed wheels, and the rolling bearing is a needleroller bearing.

[0018] The above-described transmission component is preferably arolling bearing rotatably supporting the input shaft, the output shaft,or each of the toothed wheels, and the rolling bearing is a ballbearing.

[0019] The present invention provides a method of manufacturing atransmission component incorporated into a transmission capable ofchanging a rotational speed of an output shaft relative to a rotationalspeed of an input shaft by means of mesh of toothed wheels. Thecomponent is formed at least by carbonitriding steel for a bearing'scomponent at a temperature higher than an A₁ transformation point andthen cooling the steel to a temperature lower than the A₁ transformationpoint and subsequently reheating the steel to a range of temperature ofno less than the A₁ transformation point and less than the temperatureapplied to carbo-nitride the steel, and quenching the steel.

[0020] In the present method of manufacturing the transmissioncomponent, after steel is carbonitrided the steel is cooled to atemperature of less than the A₁ transformation point before it isfinally quenched. A fine austenite grain size can be obtained and as aresult, Charpy impact, fracture toughness, anti-crack strength, fatiguelife (or rolling contact fatigue life when the component is a rollingbearing or a rolling bearing's component) and the like can be improved.

[0021] Furthermore for example by cooling to a temperature at whichaustenite transforms, austenite grain boundary in carbonitriding can beirrelevant to that in final quenching. Furthermore, the final quenchingtemperature is lower than the carbonitriding temperature, and thus theamount of un-dissolved cementite in the surface layer, which isinfluenced by the carbonitriding process, increases as compared withthat in the carbonitriding process. As such the ratio of un-dissolvedcementite increases while the ratio of austenite decreases at theheating temperature in the final quenching as compared with those ratiosin the carbonitriding process. In addition, it is seen from the Fe—Cbinary phase diagram that, in the range where cementite and austenitecoexist, the concentration of carbon in solid solution of the carbon andaustenite decreases as the quenching temperature decreases.

[0022] When the temperature is increased to the final quenchingtemperature, austenite grains are made fine since there remain a largeamount of un-dissolved cementite that prevent growth of austenitegrains. Moreover, the structure transformed from austenite to martensitethrough quenching has a low carbon concentration, so that the structurehas high toughness as compared with the structure quenched from thecarbonitriding temperature.

[0023] In the present method of manufacturing the transmissioncomponent, preferably the steel is heated to a range of temperature of790° C. to 830° C. before it is quenched.

[0024] The steel is again heated to a temperature hardly allowing anaustenite grain to be grown before the steel is quenched. Fine austenitegrain size can thus be achieved.

[0025] The present invention provides a tapered roller bearing having aninner ring, an outer ring, and a tapered roller. At least any one of theinner ring, the outer ring, and the tapered roller has a nitriding layerand an austenite grain with a grain size number falling within a rangeexceeding 10.

[0026] In the tapered roller bearing according to the present invention,at least any one of the inner ring, the outer ring and the rollingelement that provides a small austenite grain size allows significantlyincreased anti-crack strength, dimensional stability and rolling contactfatigue life. With the austenite grain size number of 10 or less, anyremarkable improvement of the rolling fatigue life is impossible andthus the grain size number is greater than 10, and preferably 11 orgreater. Although further finer austenite grains are desirable, thegrain size number exceeding 13 is usually difficult to achieve. Notethat an austenite grain of the inner ring, the outer ring, and therolling element of the support structure of the shaft in thetransmission does not vary whether it may be in a surface layer portionsignificantly affected as it is carbonitrided or a portion inner thanthe surface layer portion. As such, the surface layer portion and theinner portion will be set as positions serving as subjects of theaforementioned grain number range.

[0027] The inner and outer rings in this specification may be integratedwith a member such as a shaft or a housing, or may be providedseparately from such a member.

[0028] The austenite grain also refers to a trace thereof remainingafter the austenite is transformed into ferrite phase such as martensiteor bainite through quenching. An austenite grain boundary beforequenching is sometimes referred to as a “prior austenite grain boundary”to be distinguished from the remaining austenite grain after quenching.That is, the “austenite grain” and the “prior austenite grain boundary”are used to mean the same.

[0029] The prior austenite grain boundaries can be observed after beingsubjected to a process developing a grain boundary such as an etchingprocess for a metal phase sample of the member of interest. Formeasurement of the grain size, the average of ASTM (American Society forTesting and Materials)-defined grain size numbers (=average grain sizeof at most 8 μm may be converted to obtain an average grain diameter, orthe intercept method or the like may be used in which a straight line isplaced on a metal phase structure in an arbitrary direction to obtain anaverage length between points at which the straight line meets grainboundaries.

[0030] The above-described nitriding layer is formed by a carbonitridingprocess as will be described below. The nitriding layer may or may notbe enriched with carbon.

[0031] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic cross-sectional view of the configuration ofa transmission into which a transmission component is incorporated inaccordance with one embodiment of the present invention.

[0033]FIG. 2 is a schematic cross-sectional view of the configuration ofa deep groove ball bearing serving as rolling bearings 10A and 10B shownin FIG. 1.

[0034]FIG. 3 is a diagram for illustrating a method of a thermaltreatment applied to the transmission component in an embodiment of thepresent invention.

[0035]FIG. 4 is a diagram for illustrating an exemplary variation of themethod of the thermal treatment applied to the transmission component inthe embodiment of the present invention.

[0036]FIG. 5A shows a microstructure, more specifically, austenite grainof a bearing according to the present invention, and FIG. 5B shows amicrostructure, more specifically, austenite grain of a bearing in thebackground art.

[0037]FIG. 6A is a drawing of the austenite grain boundary shown in FIG.5A, and FIG. 6B is a drawing of the austenite grain boundary shown inFIG. 5B.

[0038]FIG. 7 is a schematic cross-sectional view of the configuration ofa tapered roller bearing.

[0039]FIG. 8 is a schematic cross-sectional view of the configuration ofa cylindrical roller bearing.

[0040]FIG. 9 is a schematic cross-sectional view of the configuration ofa needle roller bearing.

[0041]FIG. 10 is a schematic cross-sectional view of the configurationof a self-aligning roller bearing.

[0042]FIG. 11 is a schematic diagram showing the configuration in whichthe tapered roller bearing shown in FIG. 7 is utilized as a supportstructure of a shaft in a transmission.

[0043]FIG. 12 is a schematic diagram showing the configuration in whichthe self-aligning roller bearing shown in FIG. 10 is utilized as asupport structure of a shaft in a transmission.

[0044]FIG. 13 shows a sample used in a static pressure fracture strengthtest (to measure fracture stress).

[0045]FIG. 14A is a schematic front view of a rolling contact fatiguelife tester, and FIG. 14B is a schematic side view of the rollingcontact fatigue life tester.

[0046]FIG. 15 shows a sample used in a static fracture toughness test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] In the following, embodiments of the present invention will bedescribed with reference to the drawings.

[0048] Referring to FIG. 1, this transmission is a constant meshtransmission mainly including rolling bearings 10A to 10F, an inputshaft 11, an output shaft 12, a counter shaft 13, gears (toothed wheels)14 a to 14 k, and a housing 15.

[0049] Input shaft 11 is rotatably supported by housing 15 via rollingbearing 10A. Gear 14 a is provided at the outer peripheral portion ofthis input shaft 11, and gear 14 b is provided at the inner peripheralportion thereof.

[0050] One side of output shaft 12 is rotatably supported by housing 15via rolling bearing 10B, while the other side of output shaft 12 isrotatably supported by input shaft 11 via rolling bearing 10C. Thisoutput shaft 12 is provided with gears 14 c to 14 g. The axial load ofthese gears 14 c to 14 g is supported by rolling bearing 10F that is athrust needle roller bearing.

[0051] Gears 14 c and 14 d are provided at the outer and innerperipheral portions of a single member, respectively. The member withgears 14 c and 14 d is rotatably supported by output shaft 12 viarolling bearing 10D. Gear 14 e is mounted on output shaft 12 such thatit rotates with output shaft 12 and is slidable in the axial directionof output shaft 12.

[0052] Gears 14 f and 14 g are each provided at the outer peripheralportion of a single member. The member with gears 14 f and 14 g ismounted on output shaft 12 such that it rotates with output shaft 12 andis slidable in the axial direction of output shaft 12. When the memberwith gears 14 f and 14 g slides to the left in the figure, gear 14 f canmesh with gear 14 b. When the member with gears 14 f and 14 g slides tothe right in the figure, gear 14 g can mesh with gear 14 d.

[0053] Counter shaft 13 is secured to housing 15. Counter shaft 13rotatably supports a gear member having gears 14 h to 14 k or the likevia rolling bearing 10E. Gear 14 h is in constant mesh with gear 14 a,and gear 14 i is in constant mesh with gear 14 c. When gear 14 e slidesto the left in the figure, gear 14 j can mesh with gear 14 e. When gear14 e slides to the right in the figure, gear 14 k can mesh with gear 14e.

[0054] In the present embodiment, the support structure for input shaft11 has rolling bearings 10A and 10C, while the support structure foroutput shaft 12 has rolling bearings 10B and 10C. Rolling bearings 10Aand 10B are deep groove ball bearings, for example. Rolling bearing 10Cis a needle roller bearing, for example. The support structure for gears14 c and 14 h to 14 k has rolling bearings 10D to 10F. Rolling bearings10D and 10E are needle roller bearings, for example. Rolling bearing 10Fis a thrust needle roller bearing.

[0055] Referring to FIG. 2, deep groove ball bearings 10A and 10B eachhave an outer ring 1 (an outer member) secured to housing 15, an innerring 2 (an inner member) secured to input shaft 11 or output shaft 12, aplurality of balls 3 rolling between outer ring 1 and inner ring 2, anda cage 4 holding the plurality of balls 3 in place with a constant spacetherebetween.

[0056] Referring back to FIG. 1, the needle roller bearing serving asrolling bearing 10C has a cage and roller configuration in which aplurality of needle rollers 3 b are held by a cage. In thisconfiguration, an outer member of rolling bearing 10C is integrated withinput shaft 11, while an inner member thereof is integrated with outputshaft 12.

[0057] The needle roller bearings serving as rolling bearings 10D and10E each have a cage and roller configuration in which a plurality ofneedle rollers 3 b are held by a cage. In this configuration, outermembers of the rolling bearings are integrated with gears 14 c to 14 k,while inner members thereof are integrated with output shaft 12 orcounter shaft 13. The thrust needle roller bearing serving as rollingbearing 10F has an outer ring 1 c (an outer member) secured to countershaft 13, an inner ring 2 c (an inner member) secured to the gear memberhaving gears 14 h to 14 k, a plurality of needle rollers 3 c rollingbetween outer ring 1 c and inner ring 2 c, and a cage holding theplurality of needle rollers 3 c in place with a constant spacetherebetween.

[0058] The transmission component incorporated into the above-describedtransmission (for example, at least one of the outer member, the innermember, and the rolling element of rolling bearings 10A to 10F, inputshaft 11, output shaft 12, counter shaft 13, gears (toothed wheels) 14 ato 14 k, housing 15, and the like) has a nitriding layer at a surfacelayer and an austenite grain with a grain size number falling within arange exceeding 10.

[0059] Particularly when at least any one of the outer member, the innermember, and the rolling element of each of rolling bearings 10A to 10Fis the transmission component in accordance with the present embodiment,at least any one of the outer member (outer ring 1, an outer ringportion of output shaft 12, or gears 14 c, 14 h to 14 k), the innermember (inner ring 2, an inner ring portion of input shaft 11, an innerring portion of output shaft 12, or an inner ring portion of countershaft 13), and the rolling element (ball 3 or needle rollers 3 b, 3 c)includes steel having a nitriding layer and has an austenite grain witha grain size number falling within a range exceeding 10.

[0060] Furthermore, the transmission component incorporated into theabove-described transmission (for example, at least one of the outermember, the inner member, and the rolling element of rolling bearings10A to 10F, input shaft 11, output shaft 12, counter shaft 13, gears(toothed wheels) 14 a to 14 k, housing 15, and the like) has a nitridinglayer at a surface layer and a fracture stress value of at least 2650MPa.

[0061] Particularly when at least any one of the outer member, the innermember, and the rolling element of each of rolling bearings 10A to 10Fis the transmission component in accordance with the present embodiment,at least any one of the outer member (outer ring 1, the outer ringportion of output shaft 12, or gears 14 c, 14 h to 14 k); the innermember (inner ring 2, the inner ring portion of input shaft 11, theinner ring portion of output shaft 12, or the inner ring portion ofcounter shaft 13), and the rolling element (ball 3 or needle rollers 3b, 3 c) includes steel having a nitriding layer and has a fracturestress value of at least 2650 MPa.

[0062] Furthermore, the transmission component incorporated into theabove-described transmission (for example, at least one of the outermember, the inner member, and the rolling element of rolling bearings10A to 10F, input shaft 11, output shaft 12, counter shaft 13, gears(toothed wheels) 14 a to 14 k, housing 15, and the like) has a nitridinglayer at a surface layer and a hydrogen content in steel that is at most0.5 ppm.

[0063] Particularly when at least any one of the outer member, the innermember, and the rolling element of each of rolling bearings 10A to 10Fis the transmission component in accordance with the present embodiment,at least any one of the outer member (outer ring 1, the outer ringportion of output shaft 12, or gears 14 c, 14 h to 14 k), the innermember (inner ring 2, the inner ring portion of input shaft 11, theinner ring portion of output shaft 12, or the inner ring portion ofcounter shaft 13), and the rolling element (ball 3 or needle rollers 3b, 3 c) includes steel having a nitriding layer and has a hydrogencontent in steel that is at most 0.5 ppm.

[0064] In the following, a description will be given about aspeed-change operation of this transmission.

[0065] When gear 14 f does not mesh with gear 14 b, gear 14 g does notmesh with gear 14 d, and gear 14 e meshes with gear 14 j, the drivingforce of input shaft 11 is transmitted to output shaft 12 via gears 14a, 14 h, 14 j, and 14 e. The transmission in this state is in the firstgear, for example.

[0066] When gear 14 g meshes with gear 14 d and gear 14 e does not meshwith gear 14 j, the driving force of input shaft 11 is transmitted tooutput shaft 12 via gears 14 a, 14 h, 14 i, 14 c, 14 d, and 14 g. Thetransmission in this state is in the second gear, for example.

[0067] When gear 14 f meshes with gear 14 b and gear 14 e does not meshwith gear 14 j, the mesh of gears 14 b and 14 f couples input shaft 11directly to output shaft 12 so that the driving force of input shaft 11is transmitted directly to output shaft 12. The transmission in thisstate is in the third gear (high gear), for example.

[0068] In the following, a description will be given about a heattreatment including a carbonitriding process performed on thetransmission component according to the present embodiment.

[0069]FIG. 3 shows a heat treatment pattern according to which primaryquenching and secondary quenching are carried out, and FIG. 4 shows aheat treatment pattern according to which a material is cooled to atemperature lower than the A₁ transformation point in a quenchingprocess and thereafter heated again to be finally quenched. Both areexemplary embodiments of the present invention.

[0070] Referring to FIG. 3, steel for a bearing's component is firstheated to a temperature for carbonitriding (e.g. 845° C.) higher thanthe A₁ transformation point. At this temperature, the steel is subjectedto carbonitriding process. At the temperature of a treatment T₁, carbonand nitrogen are diffused in a steel matrix such that the carbon cansufficiently be included in the steel. Thereafter, at the temperature oftreatment T₁ the steel for the bearing's component is subjected to oilquenching to be cooled down to a temperature lower than the A₁transformation point. Then, the steel may be subjected to tempering at180° C. This tempering, however, may be omitted.

[0071] Thereafter, the steel is again heated to a temperature (e.g. 800°C.) of no less than the A₁ transformation point and less than thetemperature applied to carbo-nitride the steel. At this temperature, thesteel is maintained to be subjected to a treatment T₂. Then, at thetemperature of treatment T₂, the steel is subjected to oil quenching tobe cooled down to a temperature lower than the A₁ transformation point.Thereafter, the steel is subjected to tempering at 180° C.

[0072] Referring to FIG. 4, steel for a bearing's component is firstheated to a temperature for carbonitriding (e.g. 845° C.) higher thanthe A₁ transformation point. At this temperature, the steel is subjectedto carbonitriding process. At the temperature of treatment T₁, carbonand nitrogen are diffused in a steel matrix such that the carbon cansufficiently be included in the steel. Thereafter, the steel for thebearing's component is not quenched, but is cooled down to a temperatureof no more than the A₁ transformation point. Thereafter, the steel isagain heated to a temperature (e.g. 800° C.) of no less than the A₁transformation point and less than the temperature applied tocarbonitride the steel. At this temperature, the steel is maintained tobe subjected to treatment T₂. Then, at the temperature of treatment T₂,the steel is subjected to oil quenching to be cooled down to atemperature lower than the A₁ transformation point. Thereafter, thesteel is subjected to tempering at 180° C.

[0073] Compared with ordinary or normal quenching (by whichcarbonitriding is done and immediately thereafter quenching is doneonce), the above-discussed heat treatment can provide enhanced the crackstrength and reduced secular dimensional variation rate whilecarbonitriding the surface layer. This heat treatment can also produce amicrostructure having austenite crystal grains of a grain size which issmaller than the conventional one by one half or more. The transmissioncomponent subjected to the above-described heat treatment can have along fatigue life (or a long rolling contact fatigue life when thecomponent is a rolling bearing or a rolling bearing's component), anincreased anti-crack strength, and a reduced secular dimensionalvariation rate.

[0074] The above-described thermal treatments both allow theircarbonitriding processes to produce a nitriding layer that is a“carbonitriding layer.” Since the material for the carbonitridingprocess, the steel, has a high concentration of carbon, carbon in theatmosphere of the normal carbonitriding process might not enter thesurface of the steel easily. For example, with steel having a highconcentration of carbon (approximately 1% by weight), a carburized layermay have a higher concentration of carbon than this value, or acarburized layer may be formed without having a higher concentration ofcarbon than this value. A concentration of nitrogen in normal steel,however, is typically as low as approximately no more than 0.025 wt % atthe maximum although it depends on a concentration of Cr or the like.Therefore, a nitriding layer can apparently be formed regardless of theconcentration of carbon in source steel. It will be appreciated that theabove-described nitriding layer may also be enriched with carbon.

[0075]FIG. 5A shows a grain size of austenite of a bearing steel havingbeen heat-treated as shown in FIG. 3. For comparison, FIG. 5B shows agrain size of austenite of a bearing steel which has undergone theconventional heat treatment. FIGS. 6A and 6B diagrammatically show thegrain sizes of austenite that are shown in FIGS. 5A and 5B. In thestructures with the crystal grain sizes of austenite, the grain diameterof the conventional austenite is 10 which is a grain size number definedby JIS (Japanese Industrial Standard) while that of the presentinvention through the heat treatment thereof is 12 and thus fine grainsare seen. Further, the average grain diameter in FIG. 5A is 5.6 =82 mmeasured by the intercept method.

[0076] In the following, a description will be given about amodification of the support structure for the shaft in the transmission.

[0077] In the configuration shown in FIG. 1, rolling bearings 10A and10B as the support structure for the shaft were deep groove ballbearings. Rolling bearings 10A and 10B, however, may be tapered rollerbearings as shown in FIG. 7 or cylindrical roller bearings as shown FIG.8.

[0078] When at least any one of an outer member (outer ring 1), an innermember (inner ring 2), and a rolling element (roller 3) of each of thetapered roller bearing in FIG. 7 and the cylindrical roller bearing inFIG. 8 is the transmission component in accordance with the presentembodiment, the any one of the members includes steel having a nitridinglayer and has an austenite grain with a grain size number falling withina range exceeding 10.

[0079] Furthermore, when at least any one of the outer member (outerring 1), the inner member (inner ring 2), and the rolling element(roller 3) of each of the tapered roller bearing in FIG. 7 and thecylindrical roller bearing in FIG. 8 is the transmission component inaccordance with the present embodiment, the any one of the membersincludes steel having a nitriding layer and has a fracture stress valueof at least 2650 MPa.

[0080] Moreover, when at least any one of the outer member (outer ring1), the inner member (inner ring 2), and the rolling element (roller 3)of each of the tapered roller bearing in FIG. 7 and the cylindricalroller bearing in FIG. 8 is the transmission component in accordancewith the present embodiment, the any one of the members includes steelhaving a nitriding layer and has a hydrogen content in the steel that isat most 0.5 ppm.

[0081] When at least any one of the outer member (outer ring 1), theinner member (inner ring 2), and the rolling element.(roller 3) of eachof the tapered roller bearing in FIG. 7 and the cylindrical rollerbearing in FIG. 8 is the transmission component in accordance with thepresent embodiment, the any one of the members is formed by the methodas described in FIGS. 3 and 4.

[0082] The tapered roller bearing shown in FIG. 7 has, between outerring 1 and inner ring 2, a plurality of tapered rollers (conicalrollers) 3 held by cage 4. This tapered roller bearing is designed suchthat the raceway surfaces of outer ring 1 and inner ring 2 and thevertex of a cone of roller 3 meet at a point on the center line of thebearing. Therefore, the resultant of forces from the raceway surfaces ofouter ring 1 and inner ring 2 presses roller 3 against a large collar ofinner ring 2 while guiding and rolling roller 3 along the raceway.

[0083] The cylindrical roller bearing shown in FIG. 8 has, between outerring 1 and inner ring 2, a plurality of cylindrical rollers 3 held bycage 4.

[0084] The needle roller bearing shown in FIG. 9 has, between outer ring1 and an inner ring portion (not shown), a plurality of needle rollers 3held by cage 4. Roller 3 generally has a diameter of at most 5 mm and alength 3 to 10 times as long as the diameter.

[0085] The self-aligning roller bearing shown in FIG. 10 has, betweenouter ring 1 and inner ring 2, barrel-shaped rollers 3 arranged in twolines and held by cage 4. Such barrel-shaped rollers 3 arranged in twolines give a self-aligning ability capable of handling the inclinationof the shaft or the like.

[0086] Referring to FIG. 11, a shaft 21 on which a plurality of gears 24a to 24 d are provided is rotatably supported by housing 15 via taperedroller bearing 10A (or 10B).

[0087] Referring to FIG. 12, a shaft 31 on which a plurality of gears 34a to 34 d are provided is rotatably supported by housing 15 viaself-aligning roller bearing 10A.

[0088] The transmission component incorporated into the transmissionshown in FIG. 11 or 12 (for example, at least one of the plurality ofgears 24 a to 24 d, shaft 21, the outer member, the inner member, andthe rolling element of tapered roller bearings 10A and 10B, housing 15,and the like shown in FIG. 11, or at least one of the plurality of gears34 a to 34 d, shaft 31, the outer member, the inner member, and therolling element of self-aligning roller bearing 10A, housing 15, and thelike shown in FIG. 12) has a nitriding layer at a surface layer and anaustenite grain with a grain size number falling within a rangeexceeding 10.

[0089] The transmission component incorporated into the transmissionshown in FIG. 11 or 12 (for example, at least one of the plurality ofgears 24 a to 24 d, shaft 21, the outer member, the inner member, andthe rolling element of tapered roller bearings 10A and 10B, housing 15,and the like shown in FIG. 11, or at least one of the plurality of gears34 a to 34 d, shaft 31, the outer member, the inner member, and therolling element of self-aligning roller bearing 10A, housing 15, and thelike shown in FIG. 12) has a nitriding layer at a surface layer and afracture stress value of at least 2650 MPa.

[0090] The transmission component incorporated into the transmissionshown in FIG. 11 or 12 (for example, at least one of the plurality ofgears 24 a to 24 d, shaft 21, the outer member, the inner member, andthe rolling element of tapered roller bearings 10A and 10B, housing 15,and the like shown in FIG. 11, or at least one of the plurality of gears34 a to 34 d, shaft 31, the outer member, the inner member, and therolling element of self-aligning roller bearing 10A, housing 15, and thelike shown in FIG. 12) has a nitriding layer at a surface layer and ahydrogen content in the steel that is at most 0.5 ppm.

[0091] In the configuration shown in FIG. 1, rolling bearing 10C as thesupport structure for the shaft was a needle roller bearing. Rollingbearing 10C, however, may be the deep groove ball bearing as shown inFIG. 2, the tapered roller bearing as shown in FIG. 7, the cylindricalroller bearing as shown in FIG. 8, the needle roller bearing (theconfiguration having an outer ring or an inner ring) as shown in FIG. 9,or the self-aligning roller bearing as shown in FIG. 10.

[0092] The constant-mesh transmission has mainly been described in theabove embodiment. The present invention, however, is not limited to thistype of transmission. The present invention is also applicable to othertypes of transmissions such as a sliding-mesh transmission or asynchro-mesh transmission.

[0093] The present invention in examples will now be described.

EXAMPLE 1

[0094] JIS-SUJ2 (1.0 wt % of C-0.25 wt % of Si-0.4 wt % of Mn-1.5 wt %of Cr) was used for Example 1 of the present invention. Samples shown inTable 1 were each produced through the procedure described below. TABLE1 Conventionally Normally carbonitrided quenched Samples A B C D E Fproduct product Secondary 780¹⁾ 800 815 830 850 870 — — quenchingtemp.(° C.) Hydrogen — 0.37 0.40 0.38 0.42 0.40 0.72 0.38 content (ppm)Grain size No. — 12 11.5 11 10 10 10 10 (JIS) Charpy impact — 6.65 6.406.30 6.20 6.30 5.33 6.70 value (J/cm²) Fracture stress — 2840 2780 26502650 2700 2330 2770 value (MPa) Rolling contact — 5.4 4.2 3.5 2.9 2.83.1 1 fatigue life ratio (L₁₀)

[0095] Samples A-D:

EXAMPLES OF THE PRESENT INVENTION

[0096] Carbonitriding was performed at 850° C. held for 150 minutes inan atmosphere of a mixture of RX gas and ammonia gas. Following the-heattreatment pattern shown in FIG. 3, primary quenching was done from acarbonitriding temperature of 850° C., and secondary quenching wassubsequently done by heating to a temperature in a temperature rangefrom 780° C. to 830° C. lower than the carbonitriding temperature.Sample A with a secondary quenching temperature of 780° C. was nottested since quenching of sample A was insufficient.

[0097] Samples E and F:

EXAMPLES OF THE PRESENT INVENTION

[0098] These samples were carbonitrided through the same procedure asthat of samples A-D of the present invention, and then secondaryquenched at a temperature from 850° C. to 870° C. equal to or higherthan the carbonitriding temperature of 850° C.

[0099] Conventional carbonitrided sample:

COMPARATIVE EXAMPLE

[0100] carbonitriding was performed at 850° C. held for 150 minutes inan atmosphere of a mixture of RX gas and ammonia gas. Quenching wassuccessively done from the carbonitriding temperature and no secondaryquenching was done.

[0101] Normal Quenched Sample:

COMPARATIVE EXAMPLE

[0102] Without carbonitriding, quenching was done by increasing thetemperature to 850° C. and no secondary quenching was done.

[0103] For the samples above, tests were conducted for (1) measuring theamount of hydrogen, (2) measuring crystal grain size, (3) Charpy impacttest, (4) measuring fracture stress and (5) rolling fatigue test. Theirresults are shown in Table 1.

[0104] Their measuring and test methods will now be described.

[0105] (1) Measurement of Hydrogen Amount The amount of hydrogen wasdetermined by means of a DH-103 hydrogen determinator manufactured byLECO Corporation to analyze the amount of non-diffusible hydrogen in asteel. The amount of diffusible hydrogen was not measured.Specifications of the LECO DH-103 hydrogen determinator are as follows.

[0106] Analysis range: 0.01-50.00 ppm

[0107] Analysis precision: ±0.1 ppm or ±3 % H (higher one)

[0108] Analysis sensitivity: 0.01 ppm

[0109] Detection method: thermal conductimetry

[0110] Sample weight size: 10 mg-35 g (max: 12 mm (diameter)×100 mm(length))

[0111] Furnace temperature range: 50° C.-1100° C.

[0112] Reagent: anhydron (Mg(ClO₄)₂), Ascarite (NaOH)

[0113] Carrier gas: nitrogen gas, dosing gas (hydrogen gas)

[0114] (Both gases have a purity of at least 99.99% and a pressure of 40PSI (2.8 kgf/cm²).)

[0115] The procedure of the analysis is roughly described here. A samplewas taken by a dedicated sampler and the sample together with thesampler was put into the hydrogen determiner. Diffusible hydrogentherein was directed by the nitrogen carrier gas to a thermalconductimetry detector. The diffusible hydrogen was not measured in thisexample. Then, the sample was taken out of the sampler to be heated in aresistance heater and non-diffusible hydrogen was directed by thenitrogen carrier gas to the thermal conductimetry detector. The thermalconductivity was measured by the thermal conductimetry detector todetermine the amount of non-diffusible hydrogen.

[0116] (2) Measurement of Crystal Grain Size

[0117] The crystal grain size was measured according to the method oftesting the crystal grain size of austenite in a steel defined by JIS G0551.

[0118] (3) Charpy Impact Test

[0119] A Charpy impact test was conducted according to the Charpy impacttest method for a metal material defined by JIS Z 2242. A test pieceused here was a U-notch test piece (JIS No. 3 test piece) defined by JISZ 2202. Note that a Charpy impact value is a value of absorption energyE, as described below, that is divided by cross section (0.8 cm²).

[0120] Absorption energy E=WgR (cos β−cos α)

[0121] Hammer weight W=25.438 kg

[0122] Gravitational acceleration g=9.80665 n/sec²

[0123] Distance R from center of axis of rotation of hammer to center ofgravity=0.6569 m

[0124] Hammer lifted by angle α=146°

[0125] Hammer moved upward and downward by angle β

[0126] (4) Measurement of Fracture Stress

[0127]FIG. 13 shows a test piece utilized for the measurement offracture stress. Amsler universal testing machine was employed. A loadwas exerted in direction P in the figure and the load when the testpiece was fractured was measured. Then, the measured load which was afracture load was converted into a stress by the following stresscalculation formula for a curved beam. It is noted that the test pieceto be used is not limited to the one shown in FIG. 13 and may be anytest piece having a different shape.

[0128] Suppose that a fiber stress on the convex surface of the testpiece shown in FIG. 13 is σ₁ and a fiber stress on the concave surfaceis σ₂, then, σ₁ and σ₂ are determined by the following formula (JSMEMechanical Engineer's Handbook, A4-strength of materials, A4-40). Here,N indicates an axial force of a cross section including the axis of theannular test piece, A indicates a cross-sectional area, e₁ indicates anouter radius, e₂ indicates an inner radius, and κ is a section modulusof the curbed beam.

σ₁=(N/A)+{M/(Aρ _(o))}[1+e ₁{κ(ρ_(o) +e ₁)}]

σ₂=(N/A)+{M/(Aρ _(o))}[1−e ₁{κ(ρ_(o) −e ₂)}]

κ=−(1/A)∫A{η/(ρ_(o)+η)}dA

[0129] (5) Rolling Fatigue Test

[0130] The rolling fatigue life tester is shown in FIGS. 14A and 14B inits simplified form, and test conditions for a rolling fatigue life testare shown in Table 2. Referring to FIGS. 14A and 14B, a test piece 221undergoing the rolling fatigue life test was driven by a driving roll211 to rotate while being in contact with balls 213. Balls 213 were (¾)″balls guided by a guiding roll 212 to roll. Balls 213 exerted a highsurface pressure on test piece 221 while test piece 221 also exerted ahigh surface pressure on balls 213.

[0131] The results of the above-described measurements and tests willnow be described.

[0132] (1) Amount of Hydrogen

[0133] Table 1 shows that the conventional carbonitrided sample withoutbeing additionally processed has a considerably large hydrogen amount inthe steel that is 0.72 ppm. A reason therefor is considered that ammonia(NH₃) contained in the atmosphere in the carbonitriding process isdecomposed and then hydrogen enters the steel. On the other hand, thehydrogen amount in the steel of samples B-F is reduced to 0.37-0.42 ppmand thus almost a half of that of the conventional sample. This amountof hydrogen in the steel is substantially equal in level to that of thenormal quenched sample.

[0134] The above-mentioned reduction of the hydrogen amount in the steelcan lessen the degree of embrittlement of the steel that is due tohydrogen in the solid solution. In other words, by the reduction of thehydrogen amount, the Charpy impact value and the fracture stress valueof samples B-F of the present invention are remarkably improved.

[0135] (2) Crystal Grain Size

[0136] With reference to Table 1, regarding the crystal grain size,samples that are secondary quenched at a temperature lower than thequenching temperature in the carbonitriding process (primary quenching),namely samples B-D have austenite grains which are remarkably made fine,i.e., crystal grain size number is 11-12. Samples E and F as well as theconventional carbonitrided sample and the normal quenched sample haveaustenite grains with the crystal grain size number of 10, which meansthat the crystal grain size of samples E and F is greater than that ofsamples B-D.

[0137] (3) Charpy Impact Value

[0138] Table 1 shows that the Charpy impact value of the conventionalcarbonitrided sample is 5.33 J/cm² while those of samples B-F of thepresent invention are higher, ranging from 6.20 to 6.65 J/cm². It isalso seen from this that a lower secondary quenching temperature leadsto a higher Charpy impact value. The normal quenched sample has a highCharpy impact value of 6.70 J/cm².

[0139] (4) Measurement of Fracture Stress Value

[0140] The fracture stress corresponds to anti-crack strength. It isseen from Table 1 that the fracture stress of the conventionalcarbonitrided sample is 2330 MPa. On the other hand, the fracturestresses of samples B-F are improved to 2650-2840 MPa. The normalquenched sample has a fracture stress of 2770 MPa which is in the rangeof the fracture stress of samples B-F. It is considered that thereduction in hydrogen content greatly contributes to the improvedanti-crack strength of samples B-F as well as the reduction in size ofaustenite crystal grains.

[0141] (5) Rolling Fatigue Test

[0142] According to Table 1, the normal quenched sample has the shortestrolling fatigue life (L₁₀) due to the absence of a nitriding layer inthe surface layer. In contrast, the rolling fatigue life of theconventional carbonitrided sample is 3.1 times as long as that of thenormal quenched sample. The rolling fatigue life of samples B-D isremarkably improved as compared with the conventional carbonitridedsample. Samples E and F have the rolling fatigue life almost equal tothat of the conventional carbonitrided sample.

[0143] In summary, the hydrogen content in the steel can be reduced insamples B to F according to the present invention. Thus, the improvedfracture stress and Charpy impact values can be achieved in samples B toF. In addition to these fracture stress value and Charpy impact value,the rolling contact fatigue life should desirably be improved. This canbe achieved only in samples B to D having even smaller grains with anaustenite grain size number of at least approximately 11. Samples B to Fcorrespond to examples in accordance with the present invention,however, the more desirable scope of the present invention is thatcorresponding to samples B to D that have been subjected to thesecondary quenching at a temperature lower than that applied tocarbo-nitride the steel and thus has even smaller grains.

EXAMPLE 2

[0144] Example 2 of the present invention is now described.

[0145] On the following samples A, B and C, a series of tests wasconducted. A material to be heat-treated that was employed commonly tosamples A-C was JIS-SUJ2 (1.0 wt % of C-0.25 wt % of Si-0.4 wt % ofMn-1.5 wt % of Cr). Samples A-C were each processed through thefollowing procedure.

[0146] Sample A—comparative example: normal quenching only (withoutcarbonitriding)

[0147] Sample B—comparative example: quenching directly aftercarbonitriding (conventional carbonitriding and quenching)

[0148] Carbonitriding was conducted at 845° C. held for 150 minutes. Theatmosphere in the carbonitriding process was a mixture of RX gas andammonia gas.

[0149] Sample C—example of the present invention: A bearing material wasprocessed following the heat treatment pattern shown in FIG. 2.carbonitriding was conducted at 845° C. held for 150 minutes. Theatmosphere in the carbonitriding process was a mixture of RX gas andammonia gas. Final quenching temperature was 800° C.

[0150] (1) Rolling Fatigue Life

[0151] Test conditions and the test device for the rolling fatigue lifetest are as shown in Table 2 and FIG. 7. Results of the rolling fatiguelife test are shown in Table 3. TABLE 2 Test piece φ 12 × L22cylindrical test piece Number of tested pieces 10 Counterpart steel ball¾″ (19.05 mm) Contact surface pressure 5.88 GPa Load speed 46240 cpmLubricating oil Turbine VG68 - forced circulation lubrication

[0152] TABLE 3 Test Result Life (load Count) L₁₀ L₁₀ Relative Sample(×10⁴ times) (×10⁴ times) L₁₀ A 8017 18648 1.0 B 24656 33974 3.1 C 4324469031 5.4

[0153] According to Table 3, carbonitrided sample B (comparativeexample) has a rolling fatigue life (L₁₀ life: one out of ten testpieces being damaged) that is 3.1 times as long as that of sample A(comparative example) which undergoes normal quenching only, and thus itis seen that the effect of extending the life is obtained through thecarbonitriding process. In contrast, sample C of the present inventionhas a longer life which is 1.74 times as long as that of sample B and5.4 times as long as that of sample A. It is considered that thisimprovement is obtained mainly from the fine microstructure.

[0154] (2) Charpy Impact Test

[0155] A Charpy impact test was conducted by using a U-notch test piecedefined by JIS Z 2242 mentioned above. Test results are shown in Table4. TABLE 4 Charpy Impact Strength Test Charpy impact Relative Samplevalue (J/cm²) impact value A 6.7 1.0 B 5.3 0.8 C 6.7 1.0

[0156] Sample C of an example of the present invention achieved a Charpyimpact value equal to that of sample A (comparative example) havingundergone only normal quenching and higher than that of carbonitridedsample B (comparative example).

[0157] (3) Static Fracture Toughness Test

[0158] The test piece shown in FIG. 15 was used for the static fracturetoughness test. In this test piece, a pre-crack of approximately 1 mmwas made, thereafter a static load P by three-point bending was added,and then a fracture load was determined. Using the following formula, afracture toughness value (K_(Ic) value) was calculated. Results of thetest are shown in Table 5.

K _(Ic)=(PL{square root}a/BW²){5.8−9.2(a/W)+43.6(a/W)²−75.3(a/W)³+77.5(a/W)⁴} TABLE 5 Number K_(lc)Relative Sample tested (MPa{square root}m) K_(lc) A 3 16.3 1.0 B 3 16.11.0 C 3 18.9 1.2

[0159] As the previously introduced crack has a depth greater than thedepth of the nitriding layer and thus the same results are obtained forsamples A and B (comparative examples), while sample C (example of thepresent invention) achieves a fracture toughness value (K_(IC) value)approximately 1.2 times as high as those of samples A and B (comparativeexamples).

[0160] (4) Static-Pressure Fracture-Strength Test (Measurement ofFracture Stress)

[0161] A static-pressure fracture-strength test piece as shown in FIG.13 described above was used. A load was exerted in direction P in thefigure to conduct a static-pressure fracture-strength test just asdescribed above. Test results are shown in Table 6. TABLE 6 Test ResultsNumber Static fracture Relative static Sample tested strength (kgf)fracture strength A 3 4200 1.00 B 3 3500 0.84 C 3 4300 1.03

[0162] Carbonitrided sample B (comparative example) has a value of astatic-pressure fracture-strength slightly smaller than that of sample A(comparative example) having been subjected to normal quenching alone.In contrast, sample C of an example of the present invention has astatic-pressure fracture-strength value considerably higher than that ofsample B and slightly higher than that of sample A.

[0163] (5) Rate of Secular Dimensional Variation

[0164] Table 7 shows the rate of secular dimensional variation measuredunder the conditions of 130° C. (holding temperature) and 500 hours(holding time), together with the surface hardness and the amount ofretained austenite (at 0.1 mm depth from the surface). TABLE 7 Rate ofSurface dimensional Relative rate of Number hardness Retained changedimensional Sample tested (HRC) γ (%) (×10⁻⁵) change*⁾ A 3 62.5  9.0 181.0 B 3 63.6 28.0 35 1.9 C 3 60.0 11.3 22 1.2

[0165] As compared with the rate of dimensional variation of sample Bhaving a large amount of retained austenite, sample C of an example ofthe present invention has a lower rate of dimensional variation.

[0166] (6) Life Test Under Contaminated Lubricant Condition

[0167] Ball bearing 6206 was used to evaluate the rolling fatigue lifeunder a contaminated lubricant condition having a predetermined amountof normal contaminants mixed therein. Test conditions are shown in Table8 and test results are shown in Table 9. TABLE 8 Load Fr = 6.86 kNContact surface pressure Pmax = 3.2 Gpa Rate of rotation 2000 rpmLubricant Turbine 56 - oil bath lubrication Amount of contaminant 0.4g/1000 cc Contaminant Grain size: 100-180 μm, hardness: Hv800

[0168] TABLE 9 Sample L₁₀ life (h) Relative L₁₀ A 20.0 1.0 B 50.2 2.5 C74.0 3.7

[0169] Sample B (comparative example) having undergone carbonitridinghas a lifetime which is approximately 2.5 times as long as that ofsample A, and sample C of the present invention has a lifetime which isapproximately 3.7 times as long as that of sample A. While sample C ofthe present invention has a smaller amount of retained austenite thanthat of sample B of the comparative example, sample C has a longlifetime because of influences of entering nitrogen and the finemicrostructure.

[0170] It is accordingly seen from the above-discussed results that,sample C of the present invention, namely a bearing component serving asa support structure in a transmission produced by the heat treatmentmethod of the present invention can simultaneously achieve three goals:extension of the rolling fatigue life that has been difficult to achieveby the conventional carbonitriding, improvement in crack strength andreduction of the rate of secular dimensional variation.

[0171] Note that in this specification the austenite grains refer tocrystal grains of austenite which is phase-transformed during theheating process, and the traces of grains remain after the austenite istransformed into martensite through cooling.

[0172] In the transmission component and the method of manufacturing thesame and the tapered roller bearing according to the present invention,the nitriding layer is formed and in addition, a superior fracturestress value not heretofore achieved can be obtained. Therefore, theanti-crack strength or the like can be improved in the presentinvention. Additionally, the transmission can be downsized.

[0173] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims. For example, the needle rollerbearing may be a full type roller bearing or a shell type needle rollerbearing.

What is claimed is:
 1. A transmission component incorporated into a transmission capable of changing a rotational speed of an output shaft relative to a rotational speed of an input shaft by means of mesh of toothed wheels, said component having a nitriding layer at a surface layer, and an austenite grain with a grain size number falling within a range exceeding
 10. 2. The transmission component according to claim 1, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a tapered roller bearing.
 3. The transmission component according to claim 1, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a needle roller bearing.
 4. The transmission component according to claim 1, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a ball bearing.
 5. A transmission component incorporated into a transmission capable of changing a rotational speed of an output shaft relative to a rotational speed of an input shaft by means of mesh of toothed wheels, said component having a nitriding layer at a surface layer, and a fracture stress value of at least 2650 MPa.
 6. The transmission component according to claim 5, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a tapered roller bearing.
 7. The transmission component according to claim 5, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a needle roller bearing.
 8. The transmission component according to claim 5, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a ball bearing.
 9. A transmission component incorporated into a transmission capable of changing a rotational speed of an output shaft relative to a rotational speed of an input shaft by means of mesh of toothed wheels, said component having a nitriding layer at a surface layer, and a hydrogen content of at most 0.5 ppm.
 10. The transmission component according to claim 9, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a tapered roller bearing.
 11. The transmission component according to claim 9, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a needle roller bearing.
 12. The transmission component according to claim 9, provided in a form of a rolling bearing rotatably supporting said input shaft, said output shaft, or each of said toothed wheels, said rolling bearing being a ball bearing.
 13. A method of manufacturing a transmission component incorporated into a transmission capable of changing a rotational speed of an output shaft relative to a rotational speed of an input shaft by means of mesh of toothed wheels, wherein said component is formed at least by carbonitriding steel for a bearing's component at a temperature higher than an Al transformation point and then cooling the steel to a temperature lower than the Al transformation point and subsequently reheating the steel to a range of temperature of no less than the Al transformation point and less than said temperature applied to carbo-nitride the steel, and quenching the steel.
 14. The method of manufacturing the transmission component according to claim 13, wherein said range of temperature at which the quenching begins is 790° C. to 830° C.
 15. A tapered roller bearing having an inner ring, an outer ring, and a tapered roller, wherein at least any one of said inner ring, said outer ring and said tapered roller has a nitriding layer and an austenite grain with a grain size number falling within a range exceeding
 10. 